Responsible use of exotic tropical pasture cultivars — an ecological ...

Tropical Grasslands (1997) Volume 31, 332–336 332
Responsible use of exotic tropical pasture cultivars —
an ecological framework
J.G. McIVOR AND S. McINTYRE
CSIRO Tropical Agriculture, St Lucia,
Queensland, Australia
Abstract
Exotic pasture cultivars have provided considerable
economic benefits to Australia but, in recent
years, some have attracted criticism for their
environmental impacts and a number have been
labelled environmental weeds. Colonising ability
and high yields associated with the capacity to
dominate vegetation are important characteristics
of pasture cultivars as well as characteristics of
environmental weeds. Responsible use will consider
environmental impacts as well as agronomic
performance and economic benefits. This
will involve consideration of: which areas to sow;
the characteristics to select for in exotic species;
the development of native cultivars; and appropriate
management systems. Future use will
involve trade-offs and sacrifices and there will
always be some risk, but this could be reduced so
that benefits can be captured and unwanted environmental
effects minimised.
Introduction
Exotic pasture species are the basis of all sown
pastures in Australia in both temperate and
tropical regions. Although native species support
the majority of stock in northern Australia, most
have a number of deficiencies — these vary but
two are common: low quantity and/or quality of
herbage; and failure to respond positively to the
altered growing conditions under commercial
pasture development (tree clearing, cultivation at
sowing, fertiliser application, increased stocking
rate). Exotic species, which are persistent,
Correspondence: Dr J.G. McIvor, CSIRO Tropical Agriculture,
306 Carmody Rd, St Lucia, Qld 4067, Australia.
e-mail: john.mcivor@tag.csiro.au
productive and of comparatively high quality,
have been sown or become naturalised on more
than 9 M ha in northern Australia and this area
has been forecast to increase to 14 M ha by 2010
(Walker and Weston 1990; Clements 1996).
There has been considerable emphasis on the
selection of legumes, and among tropical species,
53 cultivars from 31 species have been released
through Herbage Plant Liaison Committees for
commercial use (Hacker 1997). Many exotic
grasses have also been tested, and 56 cultivars
from 30 species have been released. A number of
other species and varieties have also been, or are
being, used.
In this paper, we consider the impacts (both
positive and negative) that exotic pastures have
had, an ecological framework of factors controlling
vegetation composition, how we might
maximise the benefits from the use of exotic
species, and the scope for using native species.
Emphasis is placed on tropical species although
many of the aspects are also relevant to temperate
pastures.
Value of improved cultivars to the grazing
industries
Many authors have made estimates of the area
suitable for sown pastures in northern Australia
and the impact sowing this area might have on
the value of animal production. For example, in
Queensland, there have been estimates of 50–
60 M ha (Davies and Eyles 1965; Ebersohn and
Lee 1972; Weston et al. 1981) potentially suited
to sown pastures, although Walker and Weston
(1990) reduced these values to 22 M ha for easily
attainable pasture potential, of which over 40%
has been developed.
A recent examination (P.D. Chudleigh, personal
communication) has estimated the net
present value (5% discount rate) of the increases
in beef production over the period 1960–2020
due to exotic pasture species at over $700 M.

Responsible use of exotic tropical pasture cultivars 333
This value was determined after allowing for
costs of research, development and extension,
costs of establishing, maintaining and utilising
the pastures, and the costs of investing in
additional stock. There are additional financial
benefits to the Dairying and Wool Industries.
In addition to their value to the grazing
industries for animal production, sown species
have contributed to crop production by increasing
soil fertility during a ley phase in cropping
systems. They also provide soil cover in
orchards, and are used for revegetating bare areas
such as mine sites, roadsides, irrigation channels,
railway lines and housing developments in order
to reduce soil erosion and loss. (As discussed in
the next section, sowing exotic species in these
situations may prevent the re-establishment of
native species and lower the diversity of these
species.)
Adverse effects
Although exotic pasture species have provided
economic benefits to Australia, they have come
under increasing scrutiny in recent years and
have attracted considerable criticism for their
impacts, both where they have been sown and
particularly in areas where they have not been
sown. Lonsdale (1994) surveyed grass and
legume introductions to north-western Australia
and found that 60 of the 463 introduced species
had been listed by various authorities as weeds.
In addition to their adverse effects and associated
control costs in cropping systems and horticultural
areas, a number have been listed as
environmental weeds.
Environmental weeds are species which
invade native communities and cause changes to
the vegetation structure (species composition and
abundance), and/or the function of ecosystems.
These may result in changes to faunal habitat as
well as changes to the vegetation. Specific concerns
have been expressed about:
• changes to habitats for both plants and
animals leading to a loss of biodiversity as
native species decline or are displaced (e.g.
Brachiaria decumbens, B. mutica, Cenchrus
ciliaris);
• access (for animals and humans) prevented or
restricted (e.g. Brachiaria mutica);
• fire regimes altered, usually to more frequent
and/or intense fires (e.g. Cenchrus ciliaris,
Pennisetum polystachyon);
• drainage patterns and flow rates changed (e.g.
Brachiaria mutica); and
• landscape appearance altered with a decrease
in aesthetic values.
In areas actually sown, there are impacts due
to the sowing of the species themselves and also
associated management. Fertiliser application,
cultivation for establishment, changed grazing
regimes and timber treatment all affect biodiversity.
An example of the impacts of these
practices on species density (a measure of biodiversity)
is given in Table 1. There are also
increasing problems with soil acidification in
temperate legume-based pastures and these
problems may also occur where legumes are
introduced into tropical pastures (A. Noble,
personal communication).
Table 1. The effect of pasture management on species (total,
sown, native, exotic) density (number of species per plot) at
Hillgrove, near Charters Towers (J.G. McIvor and C.J. Gardener,
unpublished data). The values are the mean number of species
recorded in a total of fifty 0.25 m 2 quadrats in each plot during
the years 1990–1992 (6 years after the treatments were
imposed). Full details of the treatments are given in McIvor and
Gardener (1995).
Total Sown Native Exotic
Pasture
Native 39.5 2.1 34.0 3.4
Oversown 27.6 4.6 21.3 1.7
Trees
Live 33.1 3.3 26.9 3.0
Killed 33.9 3.3 28.5 2.1
Fertiliser
Nil 32.7 3.2 27.0 2.5
Superphosphate 34.4 3.4 28.3 2.6
Stocking rate (an/ha)
0.2 30.9 2.8 26.6 1.6
0.33 33.6 3.4 27.7 2.5
0.5 36.1 3.8 28.9 3.5
Two aspects of the growth of plants are
important — the ability to invade or colonise new
or unsown areas, and the ability to dominate
vegetation where they occur. Although colonising
ability is obviously important, it is the ability to
dominate vegetation and develop nearmonospecific
stands that characterises most
environmental weeds. Since these are desirable
characteristics in pasture species, there is an
obvious conflict. How can we have pasture cultivars
with characteristics which are valuable in a
pasture but which make them weeds elsewhere?

334 J.G. McIvor and S. McIntyre
It is useful to examine this dilemma in an
ecological context.
Ecological model — factors controlling
vegetation structure
Both biotic and abiotic forces are involved and
there are a number of models of how these forces
combine to influence vegetation structure. One
proposed by Grime (1977; 1979; 1988) has
received the most attention. In his model, Grime
(1979) asserts there are three factors affecting the
composition of herbaceous vegetation — competition,
stress and disturbance. These factors have
resulted in the evolution of species with combinations
of characteristics, or strategies, to enable
them to survive in their particular environments.
Stress and disturbance limit plant biomass,
and combinations of them are associated with
different plant strategies — low stress and low
disturbance with the competitive (C) strategy;
low stress and high disturbance with the ruderal
(R) strategy; and high stress and low disturbance
with the stress-tolerant (S) strategy. No viable
strategy has evolved for environments with both
high stress and high disturbance.
These strategies are associated with particular
plant characteristics (Grime 1979). For example:
Competitive — high dense leaf canopy, extensive
lateral spread, low seed production relative
to biomass, and rapid growth rate;
Ruderal — rapid growth, high seed production
relative to biomass, small stature with limited
lateral spread, and high frequency of flowering;
and
Stress-tolerant — evergreens with long-lived
leaves, slow growth rates, low palatability, and
low seed production relative to biomass.
In addition to the primary strategies (competitor,
ruderal and stress-tolerator), there are
secondary strategies (competitive ruderals [C-R],
stress-tolerant competitors [C-S], stress-tolerant
ruderals [S-R] and “C-S-R plants”) which incorporate
the characteristics of two or all three of
the primary strategies. In an analysis of New
Zealand pasture cultivars, Campbell (1990)
showed them to be C-S-R or C-R plants, and
many Australian pasture cultivars are also likely
to be in these two groups.
The balance between competition, stress and
disturbance is a major determinant of vegetation
structure and species composition at any site.
Stress tolerators (low growth rates and low reproductive
effort) are favoured in undisturbed,
unproductive land; ruderals (high growth rate,
high reproductive effort) are favoured in disturbed,
productive land; and competitors (high
growth rate, low reproductive effort) are favoured
in undisturbed, productive situations. Pasture
development can involve considerable changes to
stress and disturbance regimes and competitive
balances, and native species adapted to the predevelopment
conditions are frequently not
adapted to the altered regimes and are lost from
the vegetation after development.
Species diversity is greatest at intermediate
levels of stress and/or disturbance (Grime 1979).
At high levels of stress or disturbance, only a few
species can tolerate the extreme conditions, so
diversity is low. At low levels of stress and disturbance,
the vegetation is dominated by a few
competitive species. It is only at intermediate
levels that many species can tolerate the stress/
disturbance regime and also not be competitively
excluded.
Ruderals feature highly in lists of agricultural
weeds, but it is instructive to look at the characteristics
of the pasture species listed as environmental
weeds in Humphries et al. (1991). Eleven
tropical pasture plants are listed — nine grasses
(Andropogon gayanus, Brachiaria decumbens, B.
mutica, Cenchrus ciliaris, Echinochloa polystachya,
Hymenachne amplexicaulis, Hyparrhenia
rufa, Melinis minutiflora and Pennisetum
polystachyon) and two legumes (Leucaena leucocephala
and Stylosanthes scabra). Under appropriate
conditions, these species are capable of
forming almost mono-specific swards. Although
some of these species (e.g. Pennisetum polystachyon)
have the high colonising ability characteristic
of ruderal species, the height and lateral
spread of the competitive strategy is the most
common feature. This enables these species with
the C strategy to dominate in ungrazed situations,
but if they lack tolerance of defoliation, they may
be lost from grazed pastures (e.g. Macroptilium
atropurpureum).
Geographic/ecological regions where exotic
cultivars should not be sown
Whether there are regions where exotic cultivars
should not be sown depends on the value systems
of the decision maker — these vary widely and it

Responsible use of exotic tropical pasture cultivars 335
is difficult to come to a general conclusion. However,
when considering the use of exotic plants,
the following considerations should be made to
minimise undesirable effects:
• What is the conservation value of the area?
Riparian zones and nearby areas are attractive
for development but are also key components
of landscapes for conservation. Grassy woodlands,
particularly those on fertile soils and
with gentle topography, are particularly suited
to agricultural development, which threatens
their survival. They are poorly represented in
reserve systems and attractive to sowing due
to their responsiveness. Unconstrained development
will lead to the loss of this vegetation
type, as has occurred in southern Australia.
• Exotic species should not be introduced to
new areas where they have not previously
been used; these areas should be maintained
free of exotics.
• What is the vulnerability of the existing
vegetation to change? At what scale should
this decision be made?
• Regional, catchment and whole-property perspectives
should be taken to planning pasture
sowings, not just the individual paddock, as
effects may extend off-site well beyond the
actual sown area. At the landscape scale, there
is a need to identify threshold levels of pasture
development beyond which ecosystem
processes, including the maintenance of biodiversity,
are likely to be adversely affected.
• Exotic species are not the solution to all
problems — they should be used only where
there is an economic gain, and their use
should be avoided in other areas.
Opportunities for developing improved
“environmentally friendly” cultivars
As mentioned earlier, the problem is the invasion
and colonisation of unsown (and often ungrazed)
areas by exotic species (typical of ruderal
species) and their subsequent dominance of the
vegetation (typical of competitive species). Both
colonising ability and the high yields associated
with dominance are valuable characteristics in a
pasture plant. It may be necessary to sacrifice
these to develop “environmentally friendly”
cultivars. This could involve selecting for:
• grazing dependence — plants with medium
palatability (high palatability could lead to
loss due to selective grazing and low palatability
to dominance as other species are
lost) and lower productivity (to reduce the
dominance associated with high yields). This
can be contrasted with palatable climbing
legumes (e.g. Macroptilium atropurpureum),
which have poor persistence in grazed
pastures but which can dominate ungrazed
vegetation.
• ability to co-exist — in many situations, diet
quality will be higher from a mixture of
species in a pasture as it is unlikely one
species will be superior at all times of the year
and in all conditions. This will involve less
emphasis on selecting for high yields as this
trait can lead to dominance and suppression of
other species.
• seed characteristics — a common feature of
ruderal plants is strong seed dormancy
mechanisms and often large soil seed banks.
The use of sterile plants (e.g. Leucaena
hybrids) or plants with no seed dormancy (as
in crop plants) would greatly reduce
weediness. Long-term persistence in such
species would depend on the survival of
established perennial plants rather than their
periodic replacement.
Opportunities for management
Since a number of exotic plants are causing
environmental damage, it will be necessary to
manage areas where this is a concern. Management
can minimise spread by reducing seed production
(e.g. with grazing, mowing, fire) and
transport of seed (water, wind, animals, humans).
These measures will not prevent invasion but
they can reduce the rate of invasion. Management
should also aim to prevent establishment and
growth of exotic species by minimising disturbance
(however, all ecosystems need some disturbance
for re-establishment) and encouraging
competition from established vegetation. This
will be difficult to do except for small areas, but
has a role in critical environments. As mentioned
in other sections, farm and regional planning of
pasture sowings to minimise off-site impacts, and
the development of “user-friendly” cultivars, are
needed to complement management designed to
minimise invasion and establishment.

336 J.G. McIvor and S. McIntyre
Opportunities for developing cultivars of
native grasses and legumes
Australia has a wide diversity of native
herbaceous plants but, in contrast with the large
efforts expended on exotic pasture species, there
have been only limited efforts to develop
cultivars from native species (Waters et al. 1997).
A number of grasses (e.g. Danthonia, Themeda,
Astrebla, Microlaena, Elymus) have been studied
in southern Australia and the native legume
Glycine latifolia is on pre-release in Queensland.
Very high expectations have been held for
exotic plants to alleviate production constraints
where native plants have been considered
inadequate. In some cases, these expectations
have been too high. As noted earlier, no plant
strategies have evolved for sites with high levels
of both stress (droughted, infertile soils) and disturbance
(high grazing pressure). The exotic
“magic bullet” is not ecologically obtainable for
these sites and a change in management to lower
the stress and/or disturbance levels is the only
solution.
Development of cultivars in the future will
involve new selection criteria and it will not be
possible to focus on single traits to the exclusion
of other considerations. Trade-offs will be
necessary and in this consideration, native
species have a number of advantages:
• They have the necessary climatic and edaphic
adaptation, but suitable sowing methods and
management regimes for their persistence
need to be determined.
• There are no recognised adverse environmental
impacts of native herbaceous species.
• Differences in growth and quality between
species are frequently less than the differences
due to environmental variation, and when
native and exotic species are compared under
similar conditions, differences are often small.
• There are native species with moderate
productivity and grazing tolerance (e.g.
Heteropogon contortus, Bothriochloa and
Dichanthium species). These could form the
basis of low-input systems which may not be
as productive as high-input systems, but may
have lower risks of failure.
However, just as developing cultivars from
exotic species is a long process, developing cultivars
from native species will also take a long
time and will require a substantial effort over
many years.
Conclusions
Those actions which constitute responsible use
will depend on the value systems of the judge
and different judges will come to different conclusions.
However, in future, the potential
environmental impacts will need to be considered
as well as agronomic performance and economic
benefits. The continuing use of sown pastures
will be a balancing act and it is essential that
environmental issues are considered. There are
opportunities to develop new cultivars, both
exotic and native, and also to develop management
systems for them. This will involve tradeoffs
and sacrifices and there will always be some
risk, but this can be minimised so that their
benefits can be captured and unwanted environmental
effects prevented.
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